Is it permissible to supplicate for a betrothed woman to be made submissive to me, while also supplicating for her fiancé that Allah grant him another wife, or is this considered altering divine decree?

One should not supplicate for marriage to an engaged woman, because that entails supplicating for her separation from her fiancé, which is closer to a sin. It is more appropriate for a person to pray that Allah destine what is good for him wherever it may be, for Allah is the Knower of realities. A person may supplicate for evil while thinking it is good due to his haste. It is advisable to forget this girl and turn away from her after her engagement, as your attachment to her may ruin your life. It is forbidden to propose to a woman if another Muslim has already proposed to her before him.